1. Field of the Invention
The invention relates to a method for operating a radio transmitter sending digital messages, which radio transmitter receives high priority messages from a first source and low priority messages from a second source, wherein each high priority message is to be sent at a predefined time.
In a radio paging network with such transmitters, the global paging calls (which have highest priority) must be sent by all transmitters simultaneously to avoid interference problems between neighboring radio transmitters. Local paging calls are usually restricted to the range of only one (or a few) radio transmitter. Therefore, a radio transmitter sending a local paging call is not required to be synchronized with the other radio transmitters.
2. Description of the Prior Art
EP-A 0 536 539 describes a network where the radio transmitters are regularly switched between a local and a global transmission mode. In the global transmission mode, all transmitters are synchronized and send out the global paging calls. In the local transmission mode, each transmitter individually sends its local paging calls and is not synchronized with the other transmitters. The transmission of the global calls ends before the transmission of the local calls starts. Each transmission requires a completely new transmission sequence, independent of the previous transmissions. Furthermore, it is found that in this system there are unused gaps in the transmissions. These gaps occur because in existing standard paging codes such as POCSAG and ERMES, messages can only start at certain times, which are defined by the receiver's address. In a very large system with a variety of lengths of messages, these spaces are unusual. In small systems, however, which have only a few addresses, it is more likely that gaps occur. In systems where a few addresses are often used this problem may become severe, resulting in an inefficient use of the radio channel.